1. Field of the Invention
The present invention relates to a driver circuit and a system including a driver circuit, and particularly relates to a driver circuit having a predriver in a stage previous to an output driver and a system including such a driver circuit.
2. Background Art
FIG. 1 is a diagram showing the configuration of a related driver circuit. As shown in FIG. 1, the driver circuit includes a predriver 10 and an output driver 12.
The predriver 10 is configured by connecting plural CMOS inverters 20 in series. The output driver 12 is a CML (Current Mode Logic) type output driver and includes N-type MOS transistors N1, N2, and N3 and resistances R1 and R2.
A bias voltage BIAS is applied to the transistor N3, and hence the transistor N3 functions as a constant current source.
FIG. 2 is a diagram showing operation waveforms of the driver circuit shown in FIG. 1. As shown in FIG. 2, a node MAIN_P and a node MAIN_N which are output nodes of the predriver 10 oscillate, for example, between 0 V and a voltage VTERM.
If threshold voltages of the transistors N1 and N2 are VTHN, the transistor N1 remains off while the voltage of the node MAIN_N is between 0 V and the voltage VTHN when the node MAIN_N rises from low (0 V) to high (voltage VTERM). Accordingly, the voltage of an output terminal TX_P does not drop. Then, the voltage of the output terminal TX_P starts to drop only after the voltage of the node MAIN_N has reached VTHN.
On the other hand, the node MAIN_P drops from high (voltage VTERM) to low (0 V), and when the voltage of the node MAIN_P drops from the voltage VTERM to the voltage VTHN, the transistor N2 is turned off. Therefore, at this point, the voltage of an output terminal TX_N rises to high (voltage VTERM).
As described above, on/off timings of the transistor N1 and the transistor N2 do not coincide, whereby a voltage waveform of the output terminal TX_P and a voltage waveform of the output terminal TX_N are not perfect differential waveforms. Therefore, as shown in FIG. 2, an intersection point (VCOMMON) of the voltage waveform of the output terminal TX_P and the voltage waveform of the output terminal TX_N when the output switches between high and low has a higher potential than an intermediate potential. Namely, {(voltage of output terminal TX_P)+(voltage of output terminal TX_N)}/2=VCOMMON is not a constant value.
However, specifications needed for the driver circuit sometimes require that VCOMMON is constant (fluctuations are within a predetermined range), for example, as in the case of PCI-EXPRESS. In such specifications, it is necessary to avoid fluctuations in VCOMMON in the driver circuit as much as possible.